The invention relates to an electrochromic display cell which has a front electrode, a rear electrode and an interspaced electrolyte. More specifically, the display cell employs a vitreous carbon rear electrode which allows a fast cycle time for the display image.
Electrochromic displays are used to indicate alphanumeric characters and are generally designed as flat cells. The front electrode of a typical cell is made of a transparent front supporting surface such as glass, a transparent, electrically conductive layer such as indium oxide (In.sub.2 O.sub.3), and a layer of electrochromic material. The electrochromic layer of the front electrode contacts an electrolyte which supplies the ions required for the electrochromic reaction of the electrochromic layer. The rear electrode may have a similar structure and the working layer, which may also be an electrochromic material, likewise contacts the electrolyte. The back supporting surface of the rear electrode may, however, be made of a metal plate such as aluminum, titanium, nickel or alloy steel and an electrochromic layer may be applied thereto.
In a typical imaging configuration, the front electrode of the cell has a plurality of individual, separated segments of the conductive and electrochromic layers. Each segment has its own electrical connector and together the segments are spatially arranged so that application of an electrical potential to a selected number of them will produce a particular alphanumeric character. The conductive and electrochromic layers of the rear electrode are not typically segmented but are uninterrupted over the entire rear electrode area. Alternatively, the entire cell including the front and rear electrodes may be segmented so as to produce an imaging configuration or several unitary cells may be spatially arranged in a similar manner.
It is known that the electrochromic material for the front electrode of electrochromic cells can be made from a number of inorganic compounds, see, for example, U.S. Pat. No. 3,819,252 and U.S. Pat. No. 3,827,784. Typically, tungsten trioxide (WO.sub.3) is used. The rear electrode generally is made of tungsten trioxide or graphite in the form of paper, fabric or felt. The electrolyte may be present as a liquid, a gel or paste or as a solid. While sulfuric acid can and has been used as the electrolyte, an aprotic, non-aqueous solvent such as propylene carbonate, containing an alkali metal salt such as lithium perchlorate, LiClO.sub.4 is also used as the electrolyte. See, for example, German Offenlegungsschrift No. 30 11 506, page 1, lines 3 to 15, and page 2, lines 20 to 25.
Electrochromic cells with tungsten trioxide front and rear electrodes may be of symmetrical or asymmetrical design. In a symmetrical design, the thickness of the tungsten trioxide layers is the same, for example, about 0.5 microns. In these cells, the image shown by the front electrode is produced or erased by reversing the polarity of the voltage. In an asymmetrical design, the tungsten trioxide layer of the rear electrode is substantially thicker than that of the front electrode; see, for example, the description in German Offenlegungsschrift No. 29 06 320 or the corresponding European Pat. No. 0 015 402. In the asymmetrical design, the tungsten trioxide layer of the rear electrode is maintained at a potential which causes coloration so that those segments of the front electrode which are short-circuited to the rear electrode, likewise become colored. Such an arrangement requires a driving voltage only to erase the image.
Rear electrodes of the foregoing types, however, still have specific shortcomings. A tungsten trioxide electrode in a symmetrically constructed cell does not settle to a stable potential during cycling and therefore makes continuous operation more difficult. In addition, the low capacity of this electrode is a disadvantage. In an asymmetrical design, the rear electrode is slightly polarized, but it must first be colored by application and maintenance of a potential. Because of the manufacturing costs to perform the vapor deposition of such thick tungsten trioxide layers, the asymmetric cells are also expensive. Graphite layers, which can also be employed as rear electrodes, exhibit good electrochemical properties, but are difficult to connect to electrical contacts, do not permit a fast cycle time and are costly to integrate into electrochromic cells.